1. Field of the Invention
The present invention relates to an anode active material comprising a carbon coated with a metal or metal oxide thereon for a lithium secondary battery, a preparation method thereof, and a composite electrode and a lithium secondary battery comprising the same.
2. Description of the Prior Art
Recently, a lithium secondary battery is made with a variety of carbon materials such as cokes, graphite, etc. However, when the graphite group is used as an anode active material, change of a carbon lattice constant is large in charging and discharging, and a carbon active material is gradually separated from an electron transmission path in repeated charge and discharge. As a result, an electrode capacity is lowered. In addition, in the conventional carbon anode, conductivity is lowered due to a solid electrolyte interface (hereinafter, referred to as “SEI”) coating which is formed on the surface of the carbon active material. The “SEI coating” means a coating layer formed on the surface of the carbon active material through a reaction of lithium metal with an organic solvent when charging. The SEI coating allows passing lithium ions into the carbon structure because it has an ionic conductivity, but cuts off passage of an organic solvent. Therefore, if the SEI coating is not formed, performance of a battery is rapidly lowered because an organic solvent may penetrate into a carbon structure to cause exfoliation of the graphite structure, and accordingly, the electrode active material may separate from an ion transmission path.
However, if SEI coating is formed, electrical conductivity is low with high rate charging and discharging of a battery, because SEI coating is not electrically conductive, to result in an inhomogeneous electric potential distribution of the electrode. As a result, discharge capacity of the battery is lowered, and a cycle life of the battery is also lowered due to partial charging and discharging.
In order to complement the conductivity lowering caused by the large carbon lattice constant change of the carbon active material and the SEI coating layer formed on the surface of the carbon, it has been generally attempted to add a carbon with good conductivity, for example, acetylene black (AB) to an electrode as a conductive material. However, such solutions have not been provided yet.
In the meantime, the carbon active material may be separated from the electrode due to an organic solvent electrolyte. It may be caused by gas generation, etc. resulting from dissociation of a solvent injected into the carbon together with lithium. In order to prevent the separation of the carbon active material and improve binding strength of the active material, the amount of a binder added may be increased. However, it is also disadvantageous in that charge/discharge characteristics of the electrode are deteriorated because the amount of the electrode active material to be added is reduced and internal resistance is increased.
In a conventional secondary battery using carbon, especially graphite, as an anode active material, if an organic solvent electrolyte of a propylene carbonate (hereinafter, referred to as “PC”) group is used, cycle characteristics of an electrode are very poor and capacity of the electrode is seriously lowered. Therefore, an organic solvent electrolyte of an ethylene carbonate (hereinafter, referred to as “EC”) group is mainly used. However, even though the organic solvent electrolyte of the EC group is used, it shows not only electrode capacity lower than theoretical value but also continuous reduction of the electrode capacity in proceedings of cycles.
Therefore, some methods have been suggested for complementing the conductivity lowering due to the change of the carbon lattice constant and the SEI coating layer formed on the surface of the carbon active material. Examples of such research include the following: a method in which instead of a carbon conductive material such as AB, etc., a metal having a good conductivity such as silver is inserted into the carbon through a reductive treatment (J. Power Source, 68, pp.436-439 (1997); and Korean Society of Industrial and Engineering Chemistry, Spring General Meeting, Abstracts, pp.198-201 (1997)); and a method in which two or more anode active materials are used together by depositing tin oxide, etc. onto the carbon to improve electrode capacity (The 38th anniversary of Battery Forum, pp.207-208, Osaka, Japan, 1997).
In the latter method, unlike the present invention, carbon is added into a solution of tin chloride dissolved in distilled water, and the resulting solution is then evaporated at room and elevated temperatures, thereby to deposit the tin contained in the solution. Therefore, the deposited tin is also used as an electrode active material. In this case, although electrode capacity may be improved, high rate charge/discharge characteristics and cycle characteristics are not improved. In addition, it is disadvantageous in that an initial irreversible capacity is very high because a large amount of tin oxide is included and used as an electrode active material, and a battery capacity is lowered as a whole when LiCoO2, LiMn2O4 or the like is used as a cathode. In order to solve such problems, lithium may have to be inserted into an anode in advance by using an external resistance or external power. However, it makes a battery fabrication process complicate. It is further disadvantageous in that a capacity is continuously lowered because irreversible Li22Sn5 is produced in progress of charge/discharge.
In the mean time, the method of depositing a metal such as silver, etc. onto a carbon is for improving the conductivity of carbon electrode. That is, because silver having a good conductivity is added, a conductivity of the electrode may be improved. However, because it can not prevent the electrode capacity lowering caused by the dissociation of an organic solvent electrolyte injected together with lithium, there is a limitation on selection of an organic solvent electrolyte.
Another disadvantage of the conventional preparation method of a carbon electrode is that there are problems relating to a electrode plasticity. The most serious problem is that when an electrode is prepared by casting an electrode active material onto a metal thin plate used as a current collector, the carbon active material is separated from the collector because of weak binding strength between the collector and the active material. If a binder is added in order to solve this problem, an internal resistance of the electrode is increased, and accordingly high rate charge/discharge characteristics and cycle characteristics of the electrode are lowered. In order to enhance the binding strength between the carbon active material and the metal collector, a method for forming a metal oxide layer onto the metallic current collector is disclosed in the U.S. Pat. No. 5,616,437. In this method, the metal oxide layer is formed between the metallic current collector and carbon to enhance binding strength and conductivity between the collector and the carbon. However, this method could not solve all the problems of the conventional carbon active material.
Accordingly, it is necessary to solve all the problems in the conventional art including electrode capacity lowering caused by separation of an active material resulting from a large change of the carbon lattice constant or formation of SEI coating layer on the surface of the carbon active material, difficulties in high rate charge/discharge, and electrode capacity lowering caused by solvent dissociation reaction resulting from co-injection of an organic solvent, etc.